European patent application 0 323 857 B1 discloses a bending plate to be installed on a rotor mast, comprising a rotor mast attachment section, a rotor blade attachment section that is arranged at one free end of the bending plate and that is in the form of two bushings as well as a swing-flexible and torsion-flexible intermediate area that is situated between the rotor mast attachment section and the rotor blade attachment section and that forms a twist element. The twist element has an approximately H-shaped cross section that is formed by a centered, strip-shaped element and by two elements arranged laterally thereto, each of which has a cross section in the shape of a horizontal Y. The legs of the Y-shaped cross section are greatly spread apart. The H-shaped cross section or its combined individual elements have an essentially constant material thickness. The bending plate is made predominantly of composite fiber material. Due to the H-shaped cross section as well as the great spread of the legs of the Y-shaped cross section, an unfavorable warpage of the cross section profile can occur when the twist element is subjected to torsion.
U.S. Pat. No. 5,358,381 describes a yoke to be installed on a rotor mast, comprising a rotor mast attachment section, followed by a plate-shaped impact-flexible area, a rotor blade attachment section that is arranged at one free end of the yoke and that is in the form of two bushings as well as a swing-flexible and torsion-flexible intermediate area that is situated between the impact-flexible area and the rotor blade attachment section and that forms a twist element. The twist element has a star-shaped cross section with six flanges or arms. The flanges, which have an essentially constant material thickness, are greatly spread apart relative to each other. The yoke is made predominantly of composite fiber material. The individual flanges have an outer layer made of a fiber fabric. Due to the above-mentioned shape of the cross section as well as the great spread of the flanges or arms, an unfavorable warpage of the cross section profile can occur when the twist element is subjected to torsion.
German Published Application No. 2917301 discloses a bearingless rotor with an impact-flexible, swing-flexible and torsion-flexible structural element. The torsional flexibility gives the structural element the property of a twist element in addition to the impact-flexible and swing-flexible properties. This twist element has an essentially T-shaped or X-shaped cross section. The individual flanges or arms of the T-shaped or X-shaped cross section, which have an essentially constant thickness, are configured in the shape of slits.
For certain applications such as, for example, tilt rotors for a rotorcraft configured as a tilt rotor helicopter, it is necessary to configure the joint of a rotor blade to be as swing-stiff as possible. Due to the high loads that occur on the rotor, the twist element is required to have a high strength here. If the conventional twist elements were to be used in unchanged form for such an application purpose, this would cause the joint of a rotor blade to a rotor head to be too swing-flexible. Moreover, the strength of the twist element would no longer be adequately ensured. However, if the prior-art twist elements were designed so as to be sufficiently swing-stiff and stable, then the twist elements—due to the resultant excessively high torsional stiffness or twist stiffness—would no longer be adequately rotatable, which can have a detrimental effect especially on the controllability of a rotor equipped with such a twist element. Furthermore, such a twist element would be quite heavy and would have a very large overall diameter, which is undesirable from an aerodynamic standpoint as well as with an eye towards keeping the empty weight of a rotorcraft as low as possible. Finally, the prior-art twist elements are also very long, which is a drawback from an aerodynamic point of view since the total air resistance of the twist element is quite high and moreover, a relatively large area of the radius of the rotor cannot be used as an aerodynamically effective area of the rotor blade.